COBHAM Sea Tel 9711-56 IMA C Band, Installation Manual

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Sea Tel 9711-56 IMA Site Survey
3-5
EAR Controlled - ECCN EAR99
3.5.2. Raked Masts
Raked masts are found on vessels where the style or appearance of the entire vessel is important.
Again, the inclined mast is a direct descendant from
the masts of sailing ships – as ship owners wanted
their vessels to look more unique and less
utilitarian, they ‘raked’ the masts aft to make the
vessel appear capable of speed. This drawing
shows a raked mast, again with the preferred ADE
mounting above the radar and alternate with the
ADE below the radar.
Raked masts pose special problems in both
evaluating the mast for stiffness and mounting of
antennae. As can be seen in the drawing, all
antennae must be mounted on platforms or other
horizontal structures in order to maintain the
vertical orientation of the antenna centerline. This implies a secondary member which has a
different natural frequency than the raked mast’s natural frequency. In order to reduce the mass of
these platforms, they tend to be less stiff than the main box structure of the raked mast. Thus, they
will have lower natural frequencies than the raked mast itself. Unfortunately, the vibratory forces
will act through the stiff structure of the raked mast and excite these lighter platforms, to the
detriment of the antenna.
3.5.3. Girder Masts
Girder masts are large platforms atop a pair of columns. Just like girder constructions in buildings,
they are relatively stiff athwart ship – in their primary
axis – but less stiff longitudinally and torsionally. An
example of a girder mast is shown in this drawing, with
the preferred ADE mounting outboard and above the
radar directly on one of the columns and alternate
with the ADE centered on the girder above the plane
of the radar.
The greatest weakness of girder masts is in torsion –
where the girder beam twists about its vertical
centerline axis. As with all mast designs discussed so far, mounting the antenna in line with the
vertical support structure will reduce the vibration tendencies. Mounting the antenna directly above
the girder columns provides ample support to the antenna pedestal and locates the antenna weight
where it will influence the natural frequency of the mast the least.
3.5.4. Truss Mast
Truss masts are a variant on the girder mast concept. Rather than a pair of columns supporting a
girder beam, the construction is a framework of tubular
members supporting a platform on which the antennae
and other equipment are mounted. A typical truss mast
is shown in this photograph.
Like a girder mast, truss masts are especially stiff in the
athwart ship direction. Unlike a girder mast, the truss
can be made to be nearly as stiff in the longitudinal
direction. Truss masts are particularly difficult to
estimate the natural frequency – since a correct
modeling includes both the truss structure of the
supports and the plate/diaphragm structure of the
platform. In general, the following guidelines apply
when determining the adequate support for mounting
an antenna on a truss mast:
1. Antenna ADE pedestal gussets should align with platform stiffeners which are at least 200